From Structure Topology To Chemical Composition. XXI. Understanding The Crystal Chemistry of Barium In TS-Block Minerals

Abstract

Abstract There are 40 TS-block (Titanium–Silicate) minerals, which are divided into four groups, based on the topology and stereochemistry of the TS block. Each group of structures has a different linkage, content, and stereochemistry of Ti (+ Nb + Zr + Fe 3+ + Mg + Mn). In the crystal structures of TS-block minerals, TS blocks either link directly or alternate with I (Intermediate) blocks. All TS-block structures are characterized by a planar cell based on minimal lengths of translational vectors t 1 and t 2 , t 1 ∼ 5.4 and t 2 ∼ 7 A, and t 1 ^ t 2 ≈ 90°. Barium is an I -block cation in 20 TS minerals. In the I block, Ba atoms form a close-packed layer. Ba occurs in Groups II–IV, but does not occur in Group I where TS-blocks link directly and an I block is absent. In TS-block structures, there are three types of I block containing Ba atoms. The type-1 I block is a layer of Ba atoms parallel to the TS block with [5]-coordinated Ti in the H sheets. The type-2 I block consists of a central layer of cation polyhedra and two adjacent layers of Ba associated with [5]- and [6]-coordinated Ti in the H sheets. For the crystal structures with I blocks of type-1 and type-2, the lengths of the t 1 and t 2 vectors are t 1 ∼ 5.4 and t 2 ∼ 7 A. The type-3 I block consists of a layer of Ba or Ba (+ K + Na) atoms associated with [6]-coordinated Ti in the H sheets. The F and O atoms that are apical anions of [6]-coordinated Ti occur in the layer of Ba atoms. The crystal structure is based on two translation vectors, t 1 and t 2 ; the lengths of these vectors are doubled: 2 t 1 ∼ 10.8 and 2 t 2 ∼ 14 A. In structures with the type-3 I block, doubled minimal translations are due to distortion of rings of polyhedra in the H sheets. This distortion of rings of polyhedra is necessary to satisfy bond-valence requirements on X P M anions, where X P M = F, O. In the type-3 I block, the X P M site is coordinated by four or six cations and there is no space to accommodate an OH group plus a hydrogen bond at that site. Hydroxyl groups occur at the X O A sites in the O sheet where each X O A anion is bonded to three M O cations. Hence F atoms are ordered at the X P M sites on the periphery of the TS block and OH groups are ordered at the X O A sites in the O sheet.